The gradual transition resulted in both product lines existing at the same time. The transition from Power PC to x86 was not sudden. The shift to Intel hardware made it possible for Apple to catch up with the fast developing hardware and dropping costs of the x86 architecture, while still maintaining the trademark characteristics of Apple computers. MacBooks now use the x86 architecture that is being used in PCs, in particular, the Intel based processors and hardware. iBooks use the old Power PC architecture that has been the standard for all Apple computers. The most significant difference between the iBook laptops and MacBook laptops, is their hardware.
Although both models existed simultaneously, this was just a consequence of the transition that Apple went through at the time. The iBook is the earlier line that was to be replaced by the Macbook.
The iBook and the MacBook are separate lines of laptop computers from Apple.